The present invention provides an apparatus for metering, and automatically updating the metering by continual monitoring, the actual flow rate of an extruded material. The subject matter of this application is well-suited for use in cooperation with the subject matter of related application Ser. No. 487,069, filed on Mar. 1, 1990 by two of the applicants herein. However, it should be understood that the scope of this invention is not intended to be limited by the disclosures of the '069 application.
Accurate metering of highly viscous substances such as epoxy and putty, in variable amounts, is usually accomplished by one of two known systems: positive displacement ; and, time-based "gate" displacement. It is known that both of these conventional systems have serious drawbacks.
Commonly positive displacement metering systems utilize a material containment chamber of known dimensions, the volume of which can be controlled by means of a piston, driven by a servo or stepper motor. Since the volume of the chamber is fixed, a controlled displacement of the viscous material contained in the chamber will provide the ability to control the amount of material being displaced. Such a positive displacement apparatus usually requires a system of complex valving for properly regulating the fill cycle and the discharge cycle of the material containment chamber. Most positive displacement systems also require a servo controller and many moving components such as the hydraulic, pneumatic or motor driven piston, servo linkages, and control valves. The resultant complexity of design of positive displacement systems therefore create difficulties in controlling the accuracy of the amount of material extruded, especially when the positive displacement device is utilized over long production periods. For instance, if the materials being extruded are highly abrasive in nature, there can be excessive wear on the surfaces which come in contact with the materials being dispensed, such as the chamber, piston and valving. Other problems are encountered when the same positive displacement device is used to dispense a variety of materials in sequential operations. The device must be disassembled and cleaned between each of the dispensing cycles and the disassembly of the complex moving components of the positive displacement device may require extensive down time. A further disadvantage of cleaning such positive displacement devices results from the frequent applications of solvent to flush the positive displacement device and its moving components, which then requires proper disposal. With these disadvantages in mind, efforts have been made to reduce the undesirable complexities of material dispensing systems, thereby leading to the generation of designs of the time based "gate" displacement device.
The time based "gate" displacement device most commonly consists of a pressurized material containment chamber which feeds the viscous material to a material application nozzle. Extrusion of material from the nozzle is controlled by a gate member that opens and closes the nozzle for a predetermined amount of time. The length of time that the gate is opened and closed is usually determined from a pre-test using the viscous material. However, during the course of a long production run, the ability to obtain continued measurable accuracy of the amount of material being dispensed is dependent upon a number of variables which are known to affect the flow rate of the material passing through the nozzle in the predetermined period of time. Some of these variables include the pressure applied to the material containment chamber, the amount of viscous mass the pressure is acting against, the ambient pressure, the temperature of the material, the viscosity of the material, and any other individual factors that may influence the flow rate of a specified material. The influences of these variables on the material extrusion process are variously dependent upon the specified material in use and must, therefore, all be closely monitored and controlled to achieve continued accuracy of extrusion when utilizing the time based "gate" system. As can be imagined, the control and monitoring of these variables creates a system that in itself is difficult to calibrate and can be commonly unstable. Further, since the variables must be compensated for by operator intervention and since the variables do and will change over time, constant operator intervention is required to maintain accurate repeatability in the metering of viscous material flow.
Thus, it is an object of this invention to provide an apparatus for dispensing highly viscous putty, epoxy or other substances in repeatable, accurate amounts.
Another object of the present invention is to provide a dispensing apparatus which eliminates the mechanical complexities of positive displacement devices.
Yet another object of the present invention is to provide a dispensing apparatus which eliminates the inherent instabilities of the time based "gate" devices.